The hip flexor muscle, also known as iliopsoas, can cause severe pain if it gets tight. Its main function is to flex the thigh. The constant running and lunging on the tennis court can easily overwork the hip flexors. They get short and tight and very often will form trigger points. The hip flexors are actually two different muscles: the psoas runs on the sides of the lumbar spine to the hip, and iliacus that runs from each side of pelvis and joins the psoas at the hip. Trigger points in hip flexor refer pain into the low back, alongside the spine or across the back. It can be pretty excruciating – standing or lying down – and almost nothing can relieve it. Sometimes it is hard to get up, you might feel sciatica symptoms, and you can feel pain in the front of the thigh as well.